drivers: adc: add a driver for the CH32V003 ADC

The CH32V003 has a 8 channel, 10 bit onboard ADC. Add an immediate
mode driver and the appropriate pinctrl bindings. Note that the
CH32V003 GPIO pins have both a floating input and an analogue input
mode, and the pinctrl is needed to put the pin in analogue mode.

Signed-off-by: Michael Hope <michaelh@juju.nz>